Nuclear reactors are typically in the form of a boiling water reactor having suitable nuclear fuel disposed in a reactor pressure vessel in which water is heated. The water and steam are carried through various components and piping which are typically formed of stainless steel, with other materials such as alloy 182 weld metal and alloy 600 being used for various components directly inside the reactor pressure vessel.
Materials in the reactor core region are susceptible to irradiation assisted stress corrosion cracking. This is because the material in the core region is exposed to the highly oxidizing species generated by the radiolysis of water by both gamma and neutron radiation under normal water chemistry conditions, in addition to the effect of direct radiation assisted stress corrosion cracking. The oxidizing species increases the electrochemical corrosion potential of the material which in turn increases its propensity to undergo intergranular stress corrosion cracking or irradiation assisted stress corrosion cracking.
Suppression of the oxidizing species carried within such materials is desirable in controlling intergranular stress corrosion cracking. An effective method of suppressing the oxidizing species coming into contact with the material is to inject hydrogen into the reactor water via the feedwater system so that recombination of the oxidants with hydrogen occurs within the reactor circuit.
This method is called hydrogen water chemistry and is widely practiced for mitigating intergranular stress corrosion cracking of materials in boiling water reactors. When hydrogen water chemistry is practiced in a boiling water reactor, the electrochemical corrosion potential of the stainless steel material decreases from a positive value generally in the range of 0.050 to 0.200 V (SHE) under normal water chemistry to a value less than -0.230 V (SHE), where SHE stands for the Standard Hydrogen Electrode potential. When the electrochemical corrosion potential is below this negative value, intergranular stress corrosion cracking of stainless steel can be mitigated and its initiation can be prevented.
Considerable efforts have been made in the past decade to develop reliable electrochemical corrosion potential sensors to be used as reference electrodes which can be used to determine the electrochemical corrosion potential of operating surfaces of components.
The typical electrochemical corrosion potential sensor experiences a severe environment in view of the temperature of the water well exceeding 88xc2x0 C.; relatively high flow rates of the water up to and exceeding several m/s; and the high nuclear radiation in the core region.
A drawback of currently available sensors is that they have a limited lifetime in that some have failed after only three months of use while a few have shown evidence of operation for approximately six to nine months. Two major modes of sensor failure have been the cracking and corrosive attack in a ceramic-to-metal braze used at the sensing tip, and the dissolution of a sapphire insulating ceramic material used to electrically isolate the sensing tip from the metal conductor cable for platinum and stainless steel type sensors.
The electrochemical corrosion potential sensors may be mounted either directly in the reactor core region for directly monitoring electrochemical corrosion potential of in-core surfaces, or may be mounted outside the reactor core to monitor the electrochemical corrosion potential of out-of-core surfaces. However, the typical electrochemical corrosion potential sensor nevertheless experiences a severe operating environment in view of: the high temperature of water (typically from 250 to 300xc2x0 C. during operation and from 100 to 150xc2x0 C. during shut down); relatively high flowrates of the water up to and exceeding several m/s; and due to the high nuclear radiation in the core region. This complicates the design of the sensor since suitable materials are required for this hostile environment, and must be suitably configured for providing a water-tight assembly for a suitable useful life.
Corrosion tests in high velocity water have shown that MgO (xe2x80x9cmagnesiaxe2x80x9d), Y2O3 (xe2x80x9cyttriaxe2x80x9d) or CaO (Calcia) stabilized ZrO2 (xe2x80x9czirconiaxe2x80x9d) (xe2x80x9cMSZxe2x80x9d, xe2x80x9cYSZxe2x80x9d or xe2x80x9cCSZxe2x80x9d respectively) have a significantly lower corrosion rate than sapphire. Efforts have therefore been made to braze a platinum cap onto a stabilized zirconia insulator. Due to the characteristically high defects associated with stabilized zirconia, it has been found difficult to metallize the stabilized zirconia in order to enable effective brazing of the platinum cap thereto. Furthermore, undesirable alloy formation has been observed between the platinum in the cap and silver in the braze material.
An electrochemical corrosion potential sensor includes a ceramic tip insulating member, and a sensor tip joined to the ceramic tip insulating member, the sensor tip comprising an alloy. Further, a coating is provided on an outer surface of the sensor tip, the coating including a noble metal, and a conductor electrically connected to said sensor tip. A method for fabricating the electrochemical corrosion potential sensor is also disclosed.